The present invention relates to reflectors and, in particular, to methods for improving the abrasion-resistance of such reflectors, and, in particular, pavement markers.
Prior art reflective pavement markers have attempted to overcome the problem of abrasion caused by tire contact by several means. While the simplest solution might appear to be formation of the pavement marker reflector of an inherently abrasion-resistant material, such materials do not provide the other qualities necessary in a pavement marker reflector, such as economy and ease of manufacture, high quality optical performance, impact resistance, weatherability and resistance to chemicals encountered on the roadway and ability to withstand a wide range of in-service temperatures. Indeed, no material has been found which incorporates all of these attributes to the optimum degree. Accordingly, prior art reflective pavement markers, and in particular retrodirective cube-corner-type reflective pavement markers, have necessitated a compromise among these several desirable characteristics.
In the prior U.S. Pat. No. 3,332,327, assigned to the assignee of the present invention, there is disclosed a cube-corner reflex reflective pavement marker which is formed of a light-transmitting synthetic organic plastic resin which has good optical characteristics and impact resistance and which has a front face which is preferably disposed at a predetermined angle to the roadway surface to achieve an optimum compromise between abrasion resistance and self-cleaning or wiping of the front face by the tires of vehicular traffic. With that marker, it has been found that mechanical abrasion decreases when the angle of the front face of the pavement marker is increased, but as that angle increases, the cleaning action obtained by tire wiping on the front face of the lens decreases. Furthermore, when such markers are used in areas where, in winter months, abrasive material such as sand and salt are deliberately distributed over the roadway surface, the abrasion problem becomes particularly acute as the wiping action of the vehicle tires, combined with those abrasive materials, tends to scratch and grind the front face of the plastic lens and drastically diminish the optical effectiveness and the reflex reflective quality of the pavement marker.
Abrasion-resistant coatings which have not had the hardness of glass have been provided in the prior art for coating the surfaces of a plastic object which might be exposed to abrasive elements. But when used to coat reflective pavement markers, such coatings have either failed to provide the necessary abrasion resistance or have required curing temperatures which were so high that they distorted the plastic material of the reflector, thereby resulting in a serious deterioration of the reflector optics.
A paper entitled "Microsheet Glass Coated Plastics" by W. Hertl was published in February 1975 at the 30th Anniversary Technical Conference, Reinforced Plastics/Composites Institute, The Society of the Plastics Industry Inc., Section 9-G, Page 1, a copy of which article is filed herewith. The Hertl article discloses methods of laminating thin glass sheets to thick plastic substrates formed of Lexan and Plexiglas. But the adhesives utilized by Hertl required curing at elevated temperatures for at least several hours. Moreover, in contrast to the present invention, Hertl states that in a single side laminate, the plastic (rather than the glass) is in compression. Also, Hertl does not disclose use of a radiation curable curing system. Finally, applicants conceived use of silane and a radiation curing resin prior to publication of the Hertl article.